Presently available resolvers include exicter windings on the rotor of the resolver and additional output windings on the stator of the resolver. The rotor windings are excited with an AC field of a fixed amplitude to induce voltages in the output windings on the stator, having substantially a sinewave function of the rotor position relative to that stator. The output winding configuration may have any number of phases, the most common being two or three phases. In order to provide the excitation field, the rotor must be excited by a winding, and therefore receive electrical excitation currents throughout the rotation of such resolver. The most common technique to transfer the energy signal or excitation current is via mechanical slip rings, or additional ancillary rotary transformers.
Similarly, in most tachometer generators, signals and/or currents must also be transferred from a rotating winding to the external circuitry. Typically, the tachometers include stationary permanent magnets for field excitation and rotating armature windings with mechanical commutators upon which brushes ride for transferring the DC voltage from the rotor to the stator. Alternately, the exciting magnets may be part of the rotor, wherein poly-phase windings are uniformly distributed around the stator in a manner so as to be linked with the permanent magnet flux path through the rotor throughout the rotation of the tachometer. Such brushless DC tachometer generators require the use of additional electronic commutator circuitry, to provide the processing of the polyphase winding signals in order to drive accurate position information. A device typically used for commutation of tachometers in a Hall-effect sensor, wherein a three-phase tachometer would then require the use of three Hall-effect sensors, mounted so as to sense the location of the permanent magnet poles relative to the stator windings. However, the use of Hall-effect devices limits the temperature environments in which these tachometers may be used.
Moreover, the output waveform generated by commutated polyphase brushless tachometers with rotating magnet structures do not exhibit the desired constant voltage-velocity relationship. The non-constant relation of the polyphase output voltages relative to the rotor rotation therein includes some distortion or "ripple" caused by rotating leakage patterns from the magnets. Thus as the phases are switched according to the commutation action, typically every 60 or 90 electrical degrees, an output voltage produced contains substantial variation or ripple voltage, generally highly undesirable.